Materials Science and Engineering最新文献

The instrumented Charpy impact test, the dynamic fracture toughness test and the static fracture toughness test using the DC electrical potential method were carried out on Ti6Al4V alloys. Ti6Al4V alloys with various microstructures were used to investigate the relation between microstructural factors, including fractographic measurements, and various toughness criteria, i.e. Charpy absorbed energies (E_t, total absorbed energy; E_i, apparent crack initiation energy; E_p, apparent crack propagation energy); dynamic fracture toughness J_d; static fracture toughness (J_Ic (EPM), by DC electrical potential method or K_Q); static fracture propagation resistance curve (J_R curve); and static tearing modulus T_mat.

The toughness of the Ti6Al4V alloy as evaluated by the Charpy test was different from that given by the dynamic toughness test mainly because the microstructural unit which controlled the fracture was different in the different tests. The toughness evaluated by means of the J_R curve or T_mat does not always coincide with that given by J_Ic (EPM). Therefore it is necessary to use an appropriate toughness criterion that corresponds to the method of design. It was found that the microstructure had a greater influence on T_mat than on J_Ic (EPM). In addition, the relationships between microstructural factors (i.e. the areal fraction of the primary α phase, the mean free path in the primary α, the depth of dimples, the prior β grain size) and the various toughness criteria mentioned above must be clearly defined.

The effect of aging and partial austenitizing on the structure, mechanical properties and resistance to corrosion has been investigated in two types of age-hardenable martensitic-austenitic CrNiMo steels alloyed with titanium and aluminium.

Among the fundamental characteristics examined for these steels particular attention has been paid to metallurgical cleanness. The precipitation strengthening is caused by dispersed phases, in particular Ni₃ (Al, Ti)-γ′. During gradual overaging stabilized austenite forms on martensitic interfaces. Partial austenitizing treatment slightly above A_s produces the maximum austenite content.

Overaged 10Cr10Ni2Mo (wt.%) steel achieves superior strength coupled with a favourable level of toughness and corrosion resistance compared with CrNiMo steels of the types 13-4-1 and 15-6-1.

Within the framework of the line tension model, the “force-distance” relation is calculated for dislocations cutting extended obstacles to their glide. The treatment is based on the determination of the Gibbs free energy of activation Δ G, and includes consideration of “chemical” terms which refer to the creation of structural disorder, line energy terms which refer to the changing lengths of the dislocation inside and outside the particle and the work done by the applied effective stress $\text{ɹ}{}^{\mathrm{*}}$. The activation volume is obtained from the relation υ = −∂ΔG/∂τ^*. It turns out to be the absolute value of the Burgers vector multiplied by the total area swept by the dislocation in the particle and in the matrix during the activation event. In this paper an attempt is made to clarify the concept of the “activation distance” for extended obstacles, which in the literature is either ignored or treated incorrectly. Some examples are calculated for a rhomboidal cutting plane and different types of interaction.

The MCrAlY-type alloys used as overlay coatings have proved successful in a variety of gas turbine applications in protecting against oxidation and hot corrosion. Since the properties and behaviour of the coatings are related to the microstructure, it is essential to investigate structure formation during deposition, superalloy heat treatment and service. In this study the phase transformations during heat treatment and aging in the CoNiCrAlY-IN738LC system were investigated. The CoNiCrAlY coatings were produced by vacuum plasma spraying (VPS) in a full-scale production unit and subsequently vacuum heat-treated (at 1120°C for 2 h and at 845°C for 24 h) and aged in air at 850°C for up to 500 h. The microstructural features were revealed by scanning and transmission electron microscopy and the diffusion profiles were determined by energy dispersive X-ray analysis.

In the as-deposited condition, the CoNiCrAlY coating has a dual-phase structure of β-NiAl (B2, ordered body-centred cubic) and γ (face-centred cubic). As a consequence of interdiffusion during heat treatment at 1120°C, a three-phase equilibrium, γ + γ′ + β, forms in the interface region between the CoNiCrAlY coating and the substrate. Furthermore, yttria (Y₂O₃) and chromium carbides (M₂₃C₆) are present as dispersed particles in the CoNiCrAlY coating. It is proposed that phase transformations during heat treatment of the CoNiCrAlY-IN738LC system can be predicted by using the simplified NiCoAl three-phase equilibrium diagram.

When cork is heated in air at temperatures in the interval 100–300°C, it swells and its mass decreases. In this paper a detailed study of the effects of heat treatment on the structure, overall dimensions, mass and compression properties of cork (reproduction and virgin cork) is reported. It is observed that the lenticular channels swell while the originally corrugated cell walls become straight. These structural changes, together with the chemical alteration of the cell wall material, explain the observed effects on the mechanical properties.

It is now established that the cyclic stress—strain curve of polycrystalline wavy slip metal, e.g. copper, does not contain a plateau if the tests are carried out in conventional strain control. Since fracture research indicates that such tests homogenize the strain, and tests initiated by ramp loading appear to give higher degrees of strain localization, an investigation of cyclic response has been made in polycrystalline copper using the ramp-start method. After specimens were ramped to a desired stress level, they were then tested in strain control. The results showed a plateau to occur at 98 MPa, which corresponds to the plateau shear stress for monocrystals corrected by the Taylor factor. Observations of persistent slip band behavior on the gage surfaces, and of the dislocation structures by transmission electron microscopy (TEM) support the existence of the plateau and demonstrate that it is associated with higher degrees of strain localization induced by starting the tests by ramp loading.

Measurements of the kinetics of extrinsic grain boundary dislocations (EGBDs) when spreading and investigations of the microstructure of 0.12C22Cr17Ni austenitic steel subjected to different variants of thermal treatment are presented. Diffusional properties of grain boundaries (GB) were studied by the statistical distribution of activation energy for each state of the material. It was found that the distribution shifts towards higher values when the annealing temperature rises. The discussion deals with the structural and chemical factors responsible for the effects observed. The processes of grain growth, recovery of GB structure, secregation and precipitate dissolution have been taken into consideration.

Effects of prolonged exposures of 100–1000 h at 163°C on fatigue crack propagation and fracture toughness behavior have been studied in commercial aluminum-lithium alloy 2090-T8E41, and results compared with behavior in similarly over-aged 2124 alloy. Over-aging in 2090, which led to decreases in strength and toughness principally through the formation of plate-like copper-rich grain boundary precipitates and associated copper-depleted and δ′ precipitate-free zones, was found to result in increased fatigue crack growth rates above ∼ 10⁻⁹ m cycle⁻¹; near-threshold growth rates were less affected. Such behavior is related to a diminished role of crack-tip shielding during crack extension in over-aged microstructures, from less crack deflection and lower crack closure levels (from asperity wedging), arising from more linear crack paths. Despite this degradation in fatigue crack growth properties following high temperature exposure, crack growth rates in the over-aged 2090 remain comparable or superior to traditional high strength aluminum alloys, such as 2124-T351 and 7150-T651, due primarily to higher overall levels of crack-tip shielding.

Tension-torsion yield surfaces in 101 alloy copper tubing measured at effective strains ranging from 10⁻⁴ to 10⁻² are compared with the predictions from two polycrystalline models: the Bishop-Hill model and a uniform stress lower-bound model. Both models use the crystallite orientation distribution function (CODF) to predict anisotropy in the yield surface. Neither model predicted the yield surface well at an effective strain of 10⁻⁴. At strains above 5×10⁻⁴ both models predict the observed anisotropy between the axial and shear yield stress well. Overall, the lower-bound model prediction was found to exhibit better agreement with the measured yield surface for strains up to 10⁻². Prediction of bulk elastic constants using single crystal compliance values is also discussed. Using the CODF as a weighting factor bulk compliances were predicted to within 4% of the experimental measurements.

Thermal shock damage is typically characterized by “single quench-fracture tests” in which a specimen is quenched a single time into a water bath, then fractured. However, technical ceramics, such as engine components, solar collectors and ceramic computer memory substrates, must tolerate repeated thermal shocks under service conditions. Therefore, the understanding of damage induced by multiple thermal shocks is very important. Thermal shock damage caused by repeated thermal fatigue in yttrium iron garnet (YIG) was monitored by nondestructive elasticity measurements for each cycle of shock. For a relatively low level of shock-induced damage, an unexpected time-dependent recovery of the elastic modulus at room temperature was observed, and successfully characterized by a non-homogeneous, linear, first-order differential equation.